) I decided that he is unnaturaly knowledgeable because of his powerful Aberrant Dragonmark, which gives him the ability to psychicly extract knowledge from those around him. The problem is that I also want him to feel like he's been around a long time when he's physicly a human (or human-like species). My solution was to have his backstory involve a Rakasha try to posess him (deciding his power was best under their direct control rather then risk such a useful ability on one of their schemes) while fighting the Rakasha off, he unintentionaly used his dragonmark to fuse their minds together resulting in an unique being with the perspective of a rakasha but the ability to adapt of a mortal. The question is though, how closely is the mind and soul connected in Eberron? Is this new being an immortal or merely a mortal with an extended memory? What happens if he would die? Would he reincarnate as a Rakasha but with his human will, or would he maybe reincarnate as a new kind of being (not a deva, the character isn't particularly evil but he's no saint) should I play him similarly to a Kalashtar?
How should I proceed? I have a little time before his past becomes plot relevant but intil then I could use some input. I will answer any questions required of me in order to help out. Thanks for all the help!
Don't worry about things like alignment and the Deva race's backstory. That's behind-the-screen stuff. You're just using their stats to represent this unique individual.
As for what happens when he dies... well, two options. Both are based upon the fact that Rakshasa in Eberron are like the Quori, there is a set number of them in existance. Meaning...
1) When this NPC dies, his rakshasa half returns to the source of rakshasas and incarnates as a brand new rakshasa again, as normal for a killed rakshasa. His human half separates and goes on to Dollurh, and whatever may or may not lie beyond.
2) Their souls have become inexplicably amalgamated, and they are locked into this union forever. When the NPC dies, his dual soul reincarnates completely into another version of the NPC, taking both its rakshasa and human halves with it. The newly reincarnated form might have only vague glimpses to its former life, or may have perfect recall. Up to you.